Electrowinning process

ABSTRACT

A nickel electrowinning process carried out in a bag-free cell characterized by maintaining the electrolyte at a temperature in excess of 60*C and a pH of about 1.5 to 2.0 and feeding pregnant electrolyte to and withdrawing spent electrolyte from the cell to maintain a nickel bite of at least 5 grams per liter while operating at relatively high current densities. An important feature is inclusion of substantial amounts of boric acid in the cell electrolyte to permit nickel bites of about 10 to 15 grams per liter when operating at 85*C to 90*C. Operation without diaphragms permits use of current densities up to five times those used in conventional diaphragm cells without an increase in power requirements.

ilnited States Patent 1191 Gendlron et al.

[ Dec. 23, 1975 ELECTROWINNING PROCESS [73] Assignee: The International Nickel Company,

Inc., New York, NY.

221 Filed: Feb. 24, 1975 21 App1.No.:552,247

[30] Foreign Application Priority Data Apr. 9, 1974 Canada 197211 [52] US. Cl. 204/112 [51] Int. C1. C25C l/08 [58] Field of Search 204/1 12 [56] References Cited UNITED STATES PATENTS 2,453,757 11/1948 Renzoni 204/112 Primary Examiner-R. L. Andrews Attorney, Agent, or FirmFrancis J. Mulligan, Jr.; Ewan C. MacQueen [57] ABSTRACT A nickel electrowinning process carried out in a bagfree cell characterized by maintaining the electrolyte at a temperature in excess of 60C and a pH of about 1.5 to 2.0 and feeding pregnant electrolyte to and withdrawing spent electrolyte from the cell to maintain a nickel bite of at least grams per liter while operating at relatively high current densities. An important feature is inclusion of substantial amounts of boric acid in the cell electrolyte to permit nickel bites of about to grams per liter when operating at C to C. Operation without diaphragms permits use of current densities up to five times those used in conventional diaphragm cells without an increase in power requirements.

10 Claims, No Drawings ELECTROWINNING PROCESS The present invention is concerned with electrowinning of nickel and more particularly with electrowinning of nickel using a bag-free nickel electrowinning cell operating at reasonably large bites and low power requirements.

It is known to electrowin nickel in cells having at least one, and preferably a plurality of vertically oriented, sheet cathodes and anodes wherein each cathode is enclosed in a cathode box having a porous diaphragm (or in a bag), to isolate it from the anode or anodes. The cathode boxes and bags involve not only high capital cost in constructing a nickel electrowinning tank house but also involve high operating cost. The high operating costs arise basically out of the need to treat each cathode separately with due care to avoid tearing of diaphragms or bags as well as the need for replacing of boxes, and bags due both to tearing and to normal deterioration. The high capital cost involves not only the original provision of the cathode boxes and feeding means but also the facts (1) that a low current density of about 2 amperes per square decimeter is normally used in nickel electrowinning cells having cathode boxes and (2) that the cathode boxes take up cell space which could more profitably be used for additional anodes and cathodes. Low current density of course implies the need for a relatively large number of cells to provide a given productive capacity of a tank house.

Using cathode boxes, and low current density as practiced in normal prior art nickel electrowinning operations the art has been able to achieve a nickel bite of 15 to 30 grams per liter (gpl) but, owing to the low cathode current density the electrolyte recirculation rate has been low. If one merely removes the cathode boxes and diaphragms from the conventional nickel electrowinning operation operating at conventional temperatures, i.e., about 50C to about 60C the bite decreases to less than about 3 gpl and the efficiency of nickel deposition falls. An illustration of this is contained in the recent South African patent application No. 71/7102 filed Oct. 25, 1971 by the SEC Corporation of New Mexico, U.S.A. and naming as inventors E. H. Lowenhaupt, III et al. In this particular specification there is disclosed a nickel electrowinning operation, operating with a sulfate electrolyte containing about lOO grams per liter of nickel at a temperature of about 52 to 56C with a current density of about 3.8 amperes per square decimeter and a nickel bite of about l.5 gpl.

It is the object of the present invention to provide an electrowinning operation which avoids cathode boxes and bags (or diaphragms) while achieving a reasonably high nickel bite of greater than about 5 grams per liter and operating in the current density range of about 2 to about amperes per square decimeter.

Other objects and advantages will become apparent from the following description.

Generally speaking the present invention contemplates a process of electrowinning nickel from aqueous sulfate electrolytes comprising electrodepositing nickel at a cathode current density of about 2 to about 10 amperes per square decimeter from a nickel-containing sulfate electrolyte maintained at a temperature in excess of 60C and at a pH of about 1.5 to about 2.0 (as measured at cell temperature) and contained in a cell having no diaphragm isolating the cathode from an essentially insoluble anode and in which vigorous agitation of the electrolyte is maintained while feeding nickel-containing electrolyte (pregnant electrolyte) at a relatively high pH to said cell and withdrawing nickelcontaining electrolyte (spent electrolyte) from said cell at rates to maintain a substantially constant volume of electrolyte in said cell, to maintain the required pH in said cell and to maintain a difference in nickel content between electrolyte fed and electrolyte withdrawn of at least about 5 grams per liter.

The electrolyte usually contains at least about 40, e.g., 40 to about 130 gpl of nickel as sulfate, greater than about 0.5 gpl magnesium sulfate, up to about 150 gpl sodium sulfate, up to about gpl boric acid and is maintained at a temperature in excess of about 70C. Advantageously the magnesium sulfate content of the electrolyte is no greater than about 30 gpl and the boric acid content is at least about 5, e.g., about 5 to about 50 gpl.

For best results the operating pH of the cell is about 1.7 to about 1.9 at cell measured at operating temperature and the pH of the incoming feed electrolyte is about 5 .5 as measured at room temperature. The nickel bite, i.e., the difference in nickel concentration between the pregnant electrolyte and the spent electrolyte is advantageously about 7 to about 15 gpl.

The temperature of the cell in the novel nickel electrowinning process of the present invention is advantageously about 70C to about 90C and, even more advantageously, the temperature is about C to about C. At temperatures in the range of 85C to 90C there is achieved a greater current efficiency for nickel deposition and an increased buffer capacity of the sulfate ion-sulfuric acid equilibrium in the pH range of about 1.5 to about 2.0. This increased buffer capacity at high temperatures results in consumption of more acid (equatable to a greater nickel ion bite) for any given change of pH in the aforestated range. The buffer efficiency is also further increased by inclusion of substantial amounts of an additional buffer, e.g., boric acid in the aqueous electrolyte. At cell pH values in the range of about 1.5 to about 2, the boric acid serves to inhibit pH change with increasing hydrogen ion content rather than the prevention of the formation of nickel hydroxide which is normal function of this material in nickel plating baths of higher pH (e.g., 4 5.5 pH). For this reason it is advantageous to have at least l0 gpl of boric acid in the nickel electrowinning electrolyte. From operational point of view, it is practical when nickel electrowinning at a temperature in the range of 85C to 90C with a current efficiency of 75% to take a 12 to 15 gpl of nickel bite when about 20 to 50 gpl of boric acid is present whereas only a 7 to 8 gpl nickel bite is practical when only 5 gpl of boric acid is present.

One aspect of the present invention is that the cell operates under conditions of very efficient agitation at the cathode due to the combined effects of high temperature (low viscosity and high diffusion coefficients), agitation by anode gas and by hydrogen gas discharged at the cathode corresponding to about 25% of the total current. When operating at very high current densities, i.e., greater than about 5 amp/dm some additional means of artificial agitation may be required. Efficient agitation of the electrowinning baths, especially in the vicinity of the cathode surface, can readily be achieved using the air sparging techniques as disclosed in Canadian application Ser. No. 163,360 by the present applicant. Other techniques such as electrolyte jetting, rapid recirculation, mechanical stirring, etc., can also be used to obtain efficient agitation. The absence of cathode boxes and their diaphragms (or bags) in electrowinning cells used in carrying out the present invention not only reduces capital costs and labor but also permits interelectrode spacings of about 3 to about 5 centimeters (cm.) to provide reduction of power requirements and more anode-cathode area in a cell of a given dimension.

In addition power requirements are reduced when compared to conventional bagged or diaphragm cell operation because the ohmic losses through the diaphragm material and in addition the ohmic losses associated with the rather high pH (typically about 2.7 to 5.0) catholyte region near the cathode are eliminated in the present invention. The end result is such that current densities can be increased up to at least five fold over conventional operation without an increase in power requirements.

Commercially available, dimensionally stable anodes made of titanium and having a conductive surface layer made with a metal of the platinum group can be used with advantage as insoluble anodes. Other types of insoluble anodes which can be used in the process of the present invention include other platinum-group metal surfaces anodes, anodes having a surface of magnetic iron oxide and like materials which are not detrimentally affected by the electrolysis conditions.

Anodes made of lead or surfaced with lead dioxide are not recommended due to the possibility of lead contamination of the electrodeposited nickel. As described in copending Canadian application Ser. No. 192,1 l4 filed Feb. 8, 1974, the cathode can be roughened to an extent determined by the internal stress of the deposited nickel so as to facilitate both electrodeposition and ease of removal after electrodeposition is completed.

In order to give those skilled in the art a greater understanding and appreciation of the invention, the following examples are given:

4 sparged over the cathode and the pregnant electrolyte was fed into the cell to maintain a pH of 1.8. Nickel metal deposited under these conditions was bright and pit-free. The current efficiency was 78% and the nickel ion bite and cell voltage were 7 gpl and 2.75 volts respectively.

EXAMPLE II Using a dimensionally stable anode and a sandblasted, edgemasked titanium cathode blank (spacing 5 cm.), electrolysis was performed at a current density of 5 amp/dm with air sparging near the cathode with NiSO solution containing 80 gpl Ni, 75 gpl Na SO gpl H 80 and 5 gpl MgSO in a bag-free electrolytic cell at 85-90C.

The electrolyte at a pH of 5.5 was fed into the cell so that the cell pH did not decrease below a value of 1.6 to 1.7 during the course of a 22 hr. run. Bright and pit free Ni was electrodeposited at a current efficiency of 76.5%, the nickel bite and cell voltage, being 13.4 gpl and 2.7V respectively.

EXAMPLE 111 Using a dimensionally stable anode and a sandblasted edgemasked titanium cathode blank (spacing S cm.), electrolysis was performed at a current density of 10 amp/dm with air sparging near the cathode with NiSO solution containing 83 gpl Ni, 49.5 gpl H 80 5 gpl MgSO and gpl Na SO in a bag-free electrolytic cell at 8590C.

The electrolyte at a pH of 5.5 was fed into the cell so that the cell pH did not decrease below a value of 1.8 to 1.9 during the course of a 23 hr. run. Bright and pit free Ni was electrodeposited at a current efficiency of 78.0%, the nickel bite and cell voltage, being 11.9 gpl and 4.2 volts respectively.

Tabulated data resulting from Examples 4 to 8 is set forth in Table I under which is set forth the test conditions under which these Examples were run.

Test conditions:

Aqueous Electrolyte composition (gpl):

Temperature: Current Density:

Anode-cathode density:

Air-sparging rate (cubic feet per hour) Area of sandblasted, edgemasked) titanium cathodez) Anode:

Physical appearance of the deposits Ni: MgSO 5, (feed pH 5.5 at

5 amp/dm Dimensionally stable Anodes ln all cases the Ni was smooth, compact and pit free.

EXAMPLE 1 Smooth and compact nickel was obtained by electrolyzing aqueous NiSO solutions containing 80 gpl Ni, 5 gpl H B0 and 5 g'pl MgSO in a bagfree cell maintained at 85C. A commercial dimensionally stable anode (0.7 dm area) and a sandblasted, edgemasked Ti cathode were used at a spacing of 5 cm. During electrolysis at a current density of 5 amp/dm air was Table 1 shows that large bites of nickel, for example in excess of about 10 gpl can be taken when boric acid levels are relatively high, that is above about 20 gpl. Furthermore, the value of including some amounts of sodium sulfate in the electrolyte is shown by the slightly lower cell voltages of examples 6 and 7.

It is to be noted that if the cell temperature is permitted to drop below about 85C both the nickel bites and the current efficiencies drop from the advantageous levels set forth in Table I. At temperatures in the range of 50C to 60C the bite and current efficiency are respectively less than 2 gpl and 40% to 60%.

Although the present invention has been described in conjunction with preferred embodiments, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art will readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and appended claims.

We claim:

1. A process of electrowinning nickel from aqueous sulfate electrolytes comprising electrodepositing nickel at a cathode current density of about 2 to about 10 amperes per square decimeter from a nickel-containing sulfate electrolyte maintained at a temperature in excess of 60C and at a pH of about 1.5 to about 2.0 (as measured at cell temperature) and contained in a cell having no diaphragm isolating the cathode from an essentially insoluble anode and in which vigorous agitation of the electrolyte is maintained, while feeding nickel-containing electrolyte at a relatively high pH to said cell and withdrawing nickel containing electrolyte from said cell at rates to maintain a substantially constant volume of electrolyte in said cell, to maintain the required pH in said cell and to maintain a nickel bite of at least about 5 grams per liter.

2. A process as in claim 1 wherein the electrolyte in the cell contains about 40 to about 130 grams per liter of nickel as the sulfate, at least about 0.5 grams per liter of magnesium sulfate, up to about grams per liter of sodium sulfate and up to about 75 grams per liter of boric acid and is maintained at a temperature of at least about 70C.

3. A process as in claim 2 wherein the electrolyte in the cell is maintained at a temperature of about to about C.

4. A process as in claim 2 wherein the boric acid content of the electrolyte is about 5 to about 50 grams per liter.

5. A process as in claim 4 wherein the feed electrolyte has a pH of about 5.5 and wherein the nickel bite is from 7 to about l5 grams per liter.

6. A process as in claim 1 wherein vigorous agitation is maintained by air sparging.

7. A process as in claim 1 wherein anode to cathode spacing is about 3 to about 5 centimeters.

8. A process as in claim 2 wherein the pH of the electrolyte in the cell is about 1.7 to about 1.9.

9. A process as in claim 1 wherein vigorous agitation is maintained by electrolyte jetting, rapid recirculation, mechanical stirring and the like.

10. A process as in claim 1, employing a current agitation is provided. 

1. A PROCESS OF ELECTROWINNING NICKEL FROM AQUEOUS SULFATE ELECTROLYTES COMPRISING ELECTRODEPOSITING NICKEL TO A CATHODE CURRENT DENSITY OF ABOUT 2 TO ABOUT 10 AMPERES PER SQUARE DECIMETER FROM A NICKEL-CONTAINING SULFATE ELECTROLYTE MAINTAINED AT A TEMPERATURE IN EXCESS OF 60*C AND AT A PH OF ABOUT 1.5 TO ABOUT 2.0 (AS MEASURED AT CELL TEMPERATURE) AND CONTAINED IN A CELL HAVING NO DIAPHRAGM ISOLATING THE CATHODE FROM AN ESSENTIALLY INSOLUBLE ANODE AND IN WHICH VIGOROUS AGITATION OF THE ELECTROLYTE IS MAINTAINED, WHILE FEEDING NICKELCONTANING ELECTROLYTE AT A RELATIVELY HIGH PH TO SAID CELL AND WITHDRAWING NICKEL CONTAINING ELECTROLYTE FROM SAID CELL AT RATES TO MAINTAIN A SUBSTANTIALLY CONSTANT VOLUME OF ELECTROLYTE IN SAID CELL, TO MAINTAIN THE REQUIRED PH IN SAID CELL AND TO MAINTAIN A NICKEL BITE OF AT LEAST ABOUT 5 GRAMS PER LITER.
 2. A process as in claim 1 wherein the electrolyte in the cell contains about 40 to about 130 grams per liter of nickel as the sulfate, at least about 0.5 grams per liter of magnesium sulfate, up to about 75 grams per liter of sodium sulfate and up to about 75 grams per liter of boric acid and is maintained at a temperature of at least about 70*C.
 3. A process as in claim 2 wherein the electrolyte in the cell is maintained at a temperature of about 85 to about 90*C.
 4. A process as in claim 2 wherein the boric acid content of the electrolyte is about 5 to about 50 grams per liter.
 5. A process as in claim 4 wherein the feed electrolyte has a pH of about 5.5 and wherein the nickel bite is from 7 to about 15 grams per liter.
 6. A process as in claim 1 wherein vigorous agitation is maintained by air sparging.
 7. A process as in claim 1 wherein anode to cathode spacing is about 3 to about 5 centimeters.
 8. A process as in claim 2 wherein the pH of the electrolyte in the cell is about 1.7 to about 1.9.
 9. A process as in claim 1 wherein vigorous agitation is maintained by electrolyte jetting, rapid recirculation, mechanical stirring and the like.
 10. A process as in claim 1, employing a current density up to about 5 amp/dm2 wherein no artificial agitation is provided. 